Since a semiconductor laser which employs a ring resonator does not use a chip end face as a resonator, it is advantageous in that it is easy to apply the same to an optical integrated circuit and besides the resonance wavelength distance is so long that a single wavelength light source can be implemented readily.
Particularly in recent years, since integration with an electric circuit is easy, an optical integrated circuit on silicon is significant and, among such optical integrated circuits, a ring laser is a particularly reliable candidate as a light source.